Abstract
Nuclear poly(ADP-ribose) polymerases (PARPs), including PARPs 1, 2, and 3 and the Tankyrases, belong to a family of enzymes that can bind to chromatin and covalently modify histone- and chromatin-associated proteins with ADP-ribose derived from nuclear NAD+. The genomic loci where the nuclear PARPs bind and covalently modify chromatin are a fundamental question in PARP biology. Chromatin immunoprecipitation coupled with deep sequencing (ChIP-seq) has become an essential tool for determining specific sites of binding and modification genome-wide. Few methods are available, however, for localizing PARP-specific ADP-ribosylation events across the genome. Here we describe a variation of ChIP-seq, called Click-ChIP-seq, for identifying sites of ADP-ribosylation mediated by specific PARP family members. This method uses analog-sensitive PARP (asPARP) technology, including asPARP mutants and the alkyne-containing "clickable" NAD+ analog 8-Bu(3-yne)T-NAD+. In this assay, nuclei from cells expressing an asPARP protein of interest are incubated with 8-Bu(3-yne)T-NAD+, which is incorporated into ADP-ribose modifications mediated only by that specific asPARP protein. The nuclei are then subjected to cross-linking with formaldehyde, and the protein-linked analog ADP-ribose is clicked to biotin using copper-catalyzed alkyne-azide "click" chemistry. The chromatin is fragmented, and the fragments containing analog ADP-ribose are enriched using streptavidin-mediated precipitation. Finally, the enriched DNA is analyzed by qPCR or deep-sequencing experiments to determine which genomic loci contain ADP-ribose modifications mediated by the specific PARP protein of interest. Click-ChIP-seq has proven to be a robust and reproducible method for identifying chromatin-associated, PARP-specific ADP-ribosylation events genome-wide.